In the medical field, more and more synthetic tissue substrates are required to replace organs and tissue. This field is commonly referred to as tissue engineering. Tissue substrates that broadly correspond to the native organ or tissue as far as its biological, biochemical, biomechanical and structural properties are concerned are of particular significance. In reconstructive and regenerative medicine, traumatology and orthopedics tissue substrates in the musculo-skeletal field (a substitute for bones, cartilage and tendons) have become particularly important during the last few years. In many ways, the properties of currently available substrates do not correspond to native tissue because frequently the natural morphology is not reproduced and/or biochemical and/or biomechanical properties do not match. Tissue engineering represents a promising therapeutic approach for the repair of osteochondral defects. In this process, cells with a potential of building cartilage are incorporated into porous supporting materials and then placed in vivo into the chondral defect—directly or after in vitro pre-cultivation. For their application, these supporting materials have to meet specific requirements, particularly stability of shape, retarded decomposition, bio-compatibility, cell adherence, chondro-conductivity. The field of diseases of the joints is of particular medical and economic significance. In this medical field, the most painful conditions are those in which cartilage and the bone structure underneath have been destroyed. Cartilage has a limited ability of regeneration and normally an inferior quality compared against healthy cartilage. Numerous attempts have been made to transplant healthy cartilage and subchondral bone tissue or to keep it in cultures; however, no appropriate substitute has been created so far with such an approach.
In order to be able to use synthetic materials as appropriate substitute of cartilage numerous different properties of the natural system have to be taken into consideration. These properties include the biochemical composition, the structural identity (imitation of zonal morphology) and biomechanical properties. Different authors have suggested that porous foams on the basis of natural and synthetic polymers should be used for these materials. Mikos et al., Electronic Journal of Biotechnology, Vol. 3 No. 2, 2000 present a good overview on the technologies for the production of porous materials for tissue engineering. The authors describe several methods of producing highly porous lattice structures. Lattice structures are produced, for instance, by way of creating a three-dimensional network of fibers of poly-glycolic acid (“fiber bonding”). Another method of producing pores in a matrix includes the use of a water-soluble porogen, like, for instance, a salt. In this case, a polymer, poly-lactic acid or poly(DL-lactic-co-glycolic acid) is dissolved in chloroform or dichloromethane and then poured into a Petri culture dish filled with the porogen. During this process, the porogen diffuses into the polymer matrix. After vaporization of the solvent, the polymer/porogen composite is placed in water for two days to eliminate the porogen. The porosity of the resulting lattice can be controlled by the quantity of the porogen added while the pore size depends on the size of the porogen particles, e.g. when the porogen is a salt, the size of the salt crystals. A further method avoiding the use of organic solvents during pore formation is the use of a gas as porogen. In this process, solid sheets of a polymer are molded and then exposed to a gas, e.g. CO2, at an elevated pressure for a rather long period of time. Thus porosities of up to 93% and pore sizes up to 100 μm are obtained in which case the pores, however, are not connected with each other. Additional techniques that have been suggested for the production of porous polymer lattices are based on the concept of phase separation instead of incorporating a porogen. Such methods include emulsification/freeze drying or a liquid-liquid phase separation. In the first method, a polymer is dissolved, for instance in dichloromethane, then distilled water is added in order to form an emulsion. The polymer/water mixture is poured into a mold and quenched by introducing it into liquid nitrogen. After quenching, the lattice structures so created are freeze-dried at −55° C., which leads to the elimination of dispersed water and the polymer solvent. The liquid-liquid phase separation produces high-polymeric and low-polymeric phases within a polymer solution. The low-polymeric phase is now eliminated, leaving a highly porous polymeric network.
Yannas and collaborators propose a technique for phase separation of a hydrogel (U.S. Pat. No. 4,955,893). An aqueous suspension of collagen glycosamino glycane is frozen in a test tube in axial direction along the tube in a way that ice crystals may form. Afterwards the frozen material is exposed to a vacuum under sublimation conditions such that the ice crystals so formed sublimated and left an oriented porous channel structure suitable for the subsequent population of growing nerve cells. A similar technique is described in the U.S. Pat. No. 6,447,701 (Herschel et al.) wherein the polymer suspension is applied between two surfaces of different temperatures and wherein the surfaces oppose each other and wherein—due to the different temperatures—an essentially well-ordered or homogeneous structure of the polymer network and the crystals that are formed therein is created.
Neither technology as described in prior art has so far resulted in the production of a satisfactory substrate material that equally meets the requirements in terms of its biochemical composition, the zonal morphology and the biomechanical properties.
In accordance therewith, the present invention has been based on the problem of providing a method of producing composite materials that can be used as bone/cartilage substitute and that exhibit a tissue morphology resembling the natural tissue, that are bio-compatible and display an excellent congruence with native tissue in terms of both their biomechanical and their biochemical properties.
In addition, it has been the object of the present invention to provide a corresponding method that is easy to carry out.
Another problem of the present invention was the provision of a composite material which excellently imitates particularly the zonal morphology of natural tissue material, especially of cartilage/bone tissue.